1. Field of the Invention
The present invention relates to lithography, and more particularly, to reticle gripper devices used in lithography.
2. Background of Invention
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated, circuits, and the like. A semiconductor wafer, for example, can be used as a substrate to fabricate an integrated circuit.
During lithography, a reticle is used to transfer a desired pattern onto a substrate. The reticle is formed of a material transparent to the lithographic wavelength being used, for example, glass in the case of visible light. In other systems, such as extreme ultra violet (EUV) lithography systems, a reflective mask can be used that reflects the lithographic wavelength being used. The reticle has an image printed on it, referred to as a mask. The size of the reticle is chosen for the specific system in which it is used. A reticle six inches by six inches and one-quarter inch thick may be used, for example. During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer corresponding to the image printed on the reticle.
The projected image produces changes in the characteristics of a layer, such as a photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched, or otherwise processed, to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to process exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface of the wafer.
As should be clear from the above discussion, the accurate location and size of features produced through lithography is directly related to the precision and accuracy of the image projected onto the wafer. In particular, the rigors of sub-100 nm lithography place stringent demands not only on the lithography tool, but also on the reticle. Airborne particles and dust that settle on the mask area of a reticle can cause defects on the wafer. Small image distortions or displacements in the reticle plane can swamp critical dimension and create overlay errors.
Many approaches have been developed to reduce airborne particles and dust that settle on a mask. In particular, extreme care is taken to ensure a clean environment around the mask. For example, in addition to clean room environments and operating within a vacuum, other steps are taken to minimize contaminants, such as reticles being typically transported mask side down to minimize the settling of debris on the mask. Another approach in conventional lithography is the use of a pellicle that provides a transparent screen above the surface of a mask to prevent particles from landing on a mask. Such an approach can not be effectively used in EUV lithographic systems. In EUV applications, a mask and/or contact barrier system, such as the ones described in commonly owned, copending U.S. patent application, Ser. No. 10/628,326, filed on Jul. 29, 2003, which is incorporated herein in its entirely, can be used to reduce the number of contaminants landing on the mask portion of a reticle.
These approaches reduce, but do not eliminate the number of contaminants landing on a mask. One of the primary sources of particle contaminants and debris is the stress of contact between a reticle gripping surface and a reticle. A reticle gripper device, such as the one described in co-pending, commonly owned U.S. patent application Ser. No. 10/704,900, filed Nov. 12, 2003, which is incorporated herein in its entirety by reference, includes reticle gripping surfaces that are used to position and hold a reticle in place. Reticle gripping surfaces will touch a reticle at one or more contact points on the reticle. When the reticle gripping surfaces contact the reticle, airborne contaminants can be generated.
What is needed is a reticle gripper barrier system that reduces contaminant scattering when a reticle gripping surface contacts a reticle.